This invention relates to a method of feedback controlling the air-fuel ratio of an air-fuel mixture being supplied to internal combustion engine, and more particularly to a method of this kind which is applied immediately after the transition of the engine to the feedback control region from another operating region.
An air-fuel ratio feedback control method for internal combustion engines is already known, e.g. from Japanese Provisional Patent Publication (Kokai) No. 58-160528 owned by the assignee of the present application, which controls the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine by the use of a coefficient variable in response to the output of an oxygen concentration sensor arranged in the exhaust system of the engine during operation in an air-fuel ratio feedback control region.
This known method comprises determining whether the engine is operating in the feedback control region or in an operating region other than the former region, calculating an average value of values of the coefficient obtained during the engine operation in the feedback control region, and initiating the feedback control by using the coefficient which is set to an initial value obtained by multiplying or adding the average value by or to a predetermined value when the engine has shifted to the feedback control region from the other operating region. Thus, the initial value of the coefficient is set to an appropriate value demanded by the engine at the start of the feedback control operation, e.g. to a value enriching the air-fuel ratio of the mixture to thereby reduce the amount of NOx present in exhaust gases emitted from the engine.
However, according to the above method, the predetermined value to be multiplied by or added to the average value of the coefficient is set independently of the engine temperature, e.g. the temperature of engine coolant. As a result, the method has the following disadvantage: When the engine coolant temperature is low, the fuel to be supplied to the engine has higher viscosity than when the engine coolant temperature is high. Consequently, a great amount of fuel adheres to the inner walls of the intake pipe, which fuel is supplied to the cylinders of the engine together with fuel injected by fuel injection valves to cause the air-fuel ratio to become overrich, whereby it is difficult to restrain emission of CO and HC.